It is known that the human crystalline lens has active metabolic transport processes to maintain its health and clarity. It has been demonstrated that the crystalline lens contains sodium-potassium cellular pumps in the epithelial cells of the anterior half of the lens. These pumps are located from the anterior pole to the mid-periphery of the natural lens and it is known that the posterior half of the lens does not contain any pumps. There are sodium and potassium channels located around the entire surface of the lens. The flow of ions is necessary to maintain homeostasis in the ocular environment. A disruption in the circulation of ions throughout the crystalline lens could result in an imbalance in the system and lead to a cataract.
In some instances, when a patient encounters loss of visual acuity, implantation of an intraocular lens may be a recommended procedure. Because they facilitate less invasive surgery, soft intraocular lenses are preferred, wherever appropriate and possible. Generally, soft intraocular lenses are fabricated from two classes of materials: acrylics or silicones. Each of these materials may be hydrophilic or hydrophobic. When a material contains more than about 5% water under conditions of use, it is generally regarded as hydrophilic. Each material has found application in specific types of intraocular lenses. Common hydrogel components include N-vinyl-pyrrolidone (NVP) and hydroxyethyl methacrylate (HEMA), and the like. In some cases, the equilibrium water content appears to correlate with the permeability of the hydrogel. Thus, the higher the water content, the higher the expected ion and/or metabolite flux through the hydrogel.
In a special class of intraocular lenses, known as phakic intraocular lenses, which are implanted without removal of the natural lens, hydrogels are often preferred, especially in the case of posterior chamber phakic lenses where ion and metabolite flux is a consideration, as explained below. There are two major categories of phakic lenses: posterior chamber phakic lenses, which are implanted in the posterior chamber, just in front of the natural lens, and anterior phakic lenses which are implanted in the anterior chamber of the eye, in front of the iris.
Since phakic lenses are implanted without removal of the natural lens, the health of the natural lens must be maintained by an adequate supply of nutrients to maintain homeostasis. In the case of a posterior chamber phakic lens, the phakic lens body is interposed between the natural lens and the iris, and this may reduce or obstruct the flow of ions and/or metabolites in the vitreous to and from the natural lens. Lacking an adequate flow of ions and metabolites in the space between the phakic lens and the natural lens or through the body of the phakic lens, the risk of cataract formation increases. Because of their relatively higher ion flux as compared to hydrophobic lenses, hydrogels are the preferred lens material for posterior chamber phakic lenses. Higher ion flux through the synthetic phakic lens body permits a greater flow rate of ions and metabolites to and from the natural lens.
As pointed out above, higher ion and/or metabolite flux may in some instances correlate with high equilibrium water content. However, higher equilibrium water content typically also correlates with greater lens thickness, and thin lenses are more desirable. Accordingly, a balance must be struck between the desire for a high ion and metabolite flux through the lens and the desired lens thickness.
Accordingly, it is desirable to develop a material that has a high ion and metabolite flux, that is biocompatible and that is suitable for use as a phakic intraocular lens material. In addition, it is desirable to develop a material that has high ion and metabolite flux and relatively low water content to enable thinner intraocular lenses. Furthermore, other desirable features and characteristics of the high ion and metabolite flux lenses and materials will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.